1. Field of Invention
The present invention relates to an illumination system, a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”—direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
It is known to apply particular angular distributions to radiation incident upon the mask or reticle in order to improve the accuracy with which a pattern is projected from the patterning device onto the substrate. The angular distribution is applied to the radiation in an illumination system of the lithographic apparatus. The form of the angular distribution may be visualised and identified most easily as a spatial distribution in a pupil plane of the illumination system. Common illumination modes include annular, dipole and quadrupole.
It is desirable, for example, to provide an illumination system which is capable of forming an illumination mode in a manner which is not disclosed in the prior art.